Recently, a bidirectional communication system and a conference system where image- and audio-coded data are communicated via packets over the Internet rapidly become popular. Those systems are built around the technology called VoIP (Voice over IP) and TVoIP (TV over IP). There are several types of coding systems used for those communication systems. For example, the method of transmitting coded data, compressed by a high-efficiency compression method based on inter-frame prediction coding, is used for the moving-image coding system in many cases. In those coding systems, prediction parameters and prediction residual image data, obtained by predicting coded images from temporally preceding and following frames, are encoded to reduce the information amount of time-correlated moving image data. In addition, prediction residual image data is compressed efficiently through transform coding or quantization to allow it to be transmitted at low transmission bandwidth.
Typical compression coding standards are MPEG (Moving Picture Experts Group)-1, MPEG-2, and MPEG-4. In those compression coding standards, inter-frame prediction through motion compensation is performed for an input image frame in units of fixed-size rectangular areas called macro blocks. The signal data which is compressed by performing two-dimensional discrete cosine transform (DCT) and quantization for the resulting motion vector and prediction residual image data are encoded using variable length coding (VLC).
There are also audio (music) coding standards such as G.722, G.729, AMR-NB (Adaptive Multi Rate/Narrow Band), AMR-WB (Adaptive Multi Rate/Wide Band), MPEG-4 AAC (Advanced Audio Codec). Each of those standards can encode a predetermined amount of sampled data efficiently and transmit high-quality audio data at low transmission bandwidth.
There are many methods for delivering such image- or audio-coded data to a packet-switched IP (Internet Protocol) network. In addition, some third-generation (3GPP; 3rd Generation Partnership Project) mobile phones are used as terminals that support circuit-switched (CS) videophones. A gateway is also available for connecting those cellular phones to TVoIP clients connected to an IP network. In future, it is expected that packets will be transmitted via CS or both cellular phones and wireless base stations, to which cellular phones are connected via wireless channels, will be connected to all-IP networks with the result that IP networks and cellular phones will be connected more easily.
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